Aims: We present multiwavelength observations of one of the most distant
gamma-ray bursts detected so far, GRB 080913. Based on these observations, we
consider whether it could be classified as a short-duration GRB and discuss the
implications for the progenitor nature and energy extraction mechanisms.
Methods: Multiwavelength X-ray, near IR and millimetre observations were made
between 20.7 hours and 16.8 days after the event.
Results: Whereas a very faint afterglow was seen at the 3.5m CAHA telescope
in the nIR, the X-ray afterglow was clearly detected in both Swift and
XMM-Newton observations. An upper limit is reported in the mm range. We have
modeled the data assuming a collimated θ0≳ 3∘ blast
wave with an energy injection at 0.5 days carrying 5∼1052 erg or
approximately 12 times the initial energy of the blast wave. We find that GRB
080913 shares many of the gamma-ray diagnostics with the more recent burst GRB
090423 for being classified as short had they ocurred at low redshift. If the
progenitor were a compact binary merger, it is likely composed by a NS and BH.
The Blandford-Znajek (BZ) mechanism is the preferred one to extract energy from
the central, maximally-rotating BH. Both the magnetic field close to the event
horizon (B) and the BH mass (Mbh) are restricted within a relatively
narrow range, such that (B/3×1016G)(Mbh/7M⊙)∼1. Similar constraints on the central BH hold for collapsar-like progenitor
systems if the BZ-mechanism works for the system at hand.Comment: 7 pages, 4 figures. Accepted for publication in A&